Medical x-ray ct image display method, display device, medical x-ray ct device and reocrding medium recording program implementing this display method

ABSTRACT

A display method and apparatus of an X-ray projection image for medical use wherein a projection interested area to construct the X-ray projection image of an object is set on an image layer with an extending predetermined thickness perpendicular to an X-ray radiation plane defined by rotational radiation of an X-ray beam, the projection interested area is comprised of the three dimensional X-ray absorption coefficient data, and the X-ray projection image is produced by projecting the three dimensional X-ray absorption coefficient data existing in the projection interested area with respect to a projection plane intersecting the X-ray radiation plane.

TECHNICAL FIELD

[0001] The present invention relates to a display method of an X-rayprojection image for medical use based on three dimensional X-rayabsorption coefficient data obtained by an X-ray computed tomography(called CT hereinafter) method in which an X-ray image of an object tobe examined is obtained by turning an X-ray generator and an X-raydetector faced each other, to its display apparatus, to an X-ray CTapparatus using the display method and a recording medium for recordinga program to achieve the display method.

BACKGROUND ART

[0002] The applicants have proposed an X-ray CT method and apparatus inwhich conical X-ray beams with remarkably small sectional radiation areaare locally radiated only on a local region, a part of an object, toobtain clear three dimensional X-ray absorption coefficient data on theregion. For example, JP-A-2000-139902 discloses its CT method andapparatus.

[0003] In this laid open publication, the applicants have proposed abasic X-ray CT method in which a rotation center of a rotary arm with anX-ray generator and an X-ray detector faced each other was set on acenter of a local region of an object, conical X-ray beams are locallyradiated all around or half around a circumference of the object toobtain transmitted data, and the obtained transmitted images arebackprojected to obtain three dimensional X-ray absorption coefficientdata of the local region. Also they have proposed a method to obtain anX-ray panoramic image of a dental arch in the dental field using theabove-mentioned CT method in which a clear X-ray panoramic image couldbe obtained with a small amount of X-ray exposure and a short radiationtime.

[0004] However, such obtained X-ray panoramic image included obstacleshades such as a neck bone which existed on the radiation orbit of aconical X-ray beam other than the dental arch like a prior film-typeX-ray panoramic images. Therefore, improvement of clearness of imageshas been desired. Further, the panoramic images were generally usedimages for diagnosis in the dental field, however they weren't easilyunderstood by intuition as a dental arch was actually seen from onedirection. Although the condition of a dental root could be understoodby the obtained X-ray panoramic images, it was difficult to distinguishwhere the root exists from a cheek to a tongue.

DISCLOSURE OF THE INVENTION

[0005] The present invention is proposed to solve the above-mentionedproblems. The object of the present invention is to provide a displaymethod and apparatus for medical use for showing X-ray projection imageswhich have less obstacle shades and less X-ray exposure amount, areeasily understood by intuition for medical use, are useful for diagnosisand show where a dental root exists from a cheek to a tongue whileutilizing several advantages of local X-ray CT method and apparatus on alocal region and obtained three dimensional X-ray absorption coefficientdata. And other object of the present invention is to provide an X-rayCT apparatus for medical use using this display method and to provide arecording medium for recording a program to achieve the display method.The following items (1)-(11) propose a display method of X-rayprojection images for medical use, (12)-(21) propose a display apparatusto achieve the display method, (22)-(29) propose an X-ray CT apparatusfor medical use using the display method, and (30)-(40) propose arecording medium for recording a program to achieve the display method.

[0006] (1) This display method of X-ray projection images for medicaluse is characterized in that, without using three dimensional X-rayabsorption coefficient data obtained by an X-ray CT as it is forconstructing an X-ray projection image, three dimensional X-rayabsorption coefficient data are extracted on an image layer having apredetermined thickness in a direction perpendicular to an X-rayradiation plane, namely on the image wherein three dimensional X-rayabsorption coefficient data are seen from a rotation axial direction ofX-ray radiation and a projection interested area to construct an X-rayprojection image is set.

[0007] For example, clear border lines of all over the horizontaldirection of a dental arch aren't obtained by such image layer, however,border lines which may specify the position of the dental arch, orborder lines which may specify the position of obstacle shades such asneck bone can be obtained. Thus, only the dental arch area including thedental arch is set to be a projection interested area on the layer, thusobtaining an X-ray projection image without obstacle shades.

[0008] According to this display method, a projection plane to beprojected with an X-ray projection image is set to be a flat surfaceintersecting a radiation plane, in particular substantiallyperpendicular to an X-ray radiating direction. And an X-ray projectionimage is obtained by projecting three dimensional X-ray absorptioncoefficient data in the projection interested area with respect to theprojection plane and the obtained X-ray projection image is displayed.

[0009] In such a manner, three dimensional X-ray absorption coefficientdata existing in a direction substantially along the X-ray radiatingdirection so that clear images can be obtained. In addition, threedimensional X-ray absorption coefficient data are projected on the flatprojection plane. Therefore, exemplifying a dental arch, images seenfrom a projecting direction perpendicular to the projection plane can beobtained and they are easily comprehensive images by intuition fordental diagnosis. Further, perspectively observable image like thedental arch, namely an object to be examined, is turned by sequentiallyobtaining and displaying X-ray projection images by rotating a directionin which the X-ray projection image is to be projected, that is theprojection plane, thus obtaining highly comprehensive and highlyadvantageous images for diagnosis.

[0010] According to the method wherein the above-mentioned local X-rayCT method is applied to obtain an X-ray panoramic image of a dental archin the dental field, a rotation center of a rotary arm is fixed around amedian line inside of the dental arch and only a local region around therotation center is always locally radiated. In this case, X-rays areradiated on each tooth of the dental arch only in a limited directionalarea, namely a directional area limited like in a generally used filmtype X-ray panoramic radiation.

[0011] Therefore, when this display method is applied to the threedimensional X-ray absorption coefficient data obtained by a methodwherein the local X-ray CT method is applied to panoramic images, botheffects are multiplied so that several advantages of local X-ray CTmethod and apparatus can be used.

[0012] (2) Comparing with the method (1), this display method of X-rayprojection images for medical use is characterized in that not only oneX-ray projection image obtained by the method (1) is shown but alsoplural X-ray projection images can be shown in array. Therefore, inaddition to the effects of (1), X-ray projection images in which anobject such as a dental arch is seen from different directions can becompared each other and be compared in a list to select an imagerequired for diagnosis because of such plural displaying, thereforebeing convenient.

[0013] (3) Comparing with the method (1), this display method of X-rayprojection images for medical use can show the object in a rotationalmanner (in a manner that the object is rotated) by continuously showingthe X-ray projection images obtained by the method (1) while changingthe projecting directions. Therefore, in addition to the effects of (1),even if the display screen is limited, the X-ray projection images of anobject such as a dental arch can be continuously compared to select anecessary image for diagnosis because of the continuous rotary displayof the X-ray projection images which are easily comprehended byintuition. Accordingly, this method is convenient.

[0014] (4) Comparing with the method (1), this display method of X-rayprojection images for medical is characterized in that the projectioninterested area is in advance divided into several projection interestedlayers neighboring each other considering an X-ray radiating direction,three dimensional X-ray absorption coefficient data in optional onelayer or the neighboring plural layers are used and thus obtained X-rayprojection images are selectively displayed. Therefore, in addition tothe effects of (1), if the projection interested area is a dental archarea, the projection interested layer is a layer neighboring each otherfrom a cheek to a tongue. Thus knowing the image layer used forconstructing the X-ray projection image of the dental root, the positionof the root where in a cheek to a tongue can be understood.

[0015] (5) Comparing with the method (1), this display method of X-rayprojection images for medical use combines the display in array in (2),the continuous display in (3) and the display method in (4) wherein theprojection interested area is divided into plural projection interestedlayers and the X-ray projection images obtained by using the threedimensional X-ray absorption coefficient data in the projectioninterested layer are selectively shown. Therefore, the effects of (2),(3) and (4) are multiplied in addition to the effect of (1).

[0016] (6) This display method of X-ray projection images for medicaluse particularly defines the projection interested area of theabove-mentioned display methods into a dental arch area. Therefore, theabove-mentioned effects can be achieved for displaying the X-rayprojection image of the dental arch area.

[0017] (7) According to this display method of X-ray projection imagesfor medical use, in case that the projection interested area is a dentalarch area, the projection plane is arranged to be parallel to a risingdirection of a tooth or a projecting direction of a dental root in thedental arch area. The rising direction of a tooth isn't always adirection perpendicular to an articulation surface of the dental arch,namely a direction orthogonal to a projecting direction of X-ray.Therefore, if a projection plane is normally set, an X-ray projectionimage showing a rising direction of a dental tooth at an angle isobtained and the accurate length of the tooth in a rising directionisn't shown in the image. However, if the projection plane is parallelto the rising direction of the tooth, the rising length of the tooth isaccurately shown on the image, thus improving convenience.

[0018] (8) In this display method of X-ray projection images for medicaluse, when the projection interested area is a dental arch area, arotation center of a projection plane which is rotatively moved isfixed. In such a manner, even in a method a local X-ray CT method isapplied to panoramic images, when a rotation center of a rotary arm isfixed at the time of projection, control is facilitated. Otherwise, arotary arm may be moved in case of projection.

[0019] (9) In this display method of X-ray projection images for medicaluse, a rotation center of the projection plane which is rotatively movedis transferred in a predetermined pattern in the above-mentioned displaymethods (5)-(7) wherein the projection interested area is a dental archarea. In this case, if a rotation center of a rotary arm isn't fixedduring projection when the local X-ray CT method is applied to panoramicimages, clear X-ray projection images can be obtained by conformingradiating conditions and projecting conditions. When a rotation centerof a rotary arm is fixed during projection, obstacle shades such as aneck bone can be eliminated.

[0020] (10) In this display method of X-ray projection images formedical use, a method characterized in that a projection interested areais set and three dimensional X-ray absorption coefficient data in thearea are projected is applied to produce X-ray panoramic images.Therefore, images without obstacle shades are also obtained for X-raypanoramic images.

[0021] The projection plane for producing X-ray panoramic images iscurved unlike the flat projection plane in (1). If an object is a dentalarch, the curved projection plane is a curved plane binding the centerof each tooth. These X-ray panoramic images aren't only used for adental arch in the dental field, but they include X-ray projectionimages used for other medical field such as diagnosis of rib bone inaddition to an otolaryngology area, a dental surgery area, and a maxillofacial area to obtain images by sequentially projecting on the curvedprojection plane.

[0022] (11) In this display method of X-ray projection images formedical use, the display method characterized in that a projectioninterested area is divided into projection interested layers and theX-ray projection images obtained by three dimensional X-ray absorptioncoefficient data in the projection interested layer are selectivelydisplayed is applied to X-ray panoramic images. Therefore, as to X-raypanoramic images, where a dental root exists from a cheek to a tonguecan be understood.

[0023] (12) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display methods (1) and(2). Therefore, it has the same effect as (1) and (2). Further, theX-ray projection images which have been once stored are read out to bedisplayed so that the X-ray projection images aren't required to beproduced each time. Therefore, the X-ray projection images can bepromptly displayed, thus preventing an operator from being annoyed towait for a display required for diagnosis.

[0024] (13) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display method (3).Therefore, it has the same effect as (3). Further, the X-ray projectionimages which have been once stored are read out to be displayed so thatthe X-ray projection images aren't required to be produced each time.Therefore, the X-ray projection images can be promptly displayed, thuspreventing an operator from being annoyed to wait for display requiredfor diagnosis.

[0025] (14) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display method (4).Therefore, it has the same effect as (4). Further, the X-ray projectionimages which have been once stored are read out to be displayed so thatthe X-ray projection images aren't required to be produced each time.Therefore, the X-ray projection images can be promptly displayed, thuspreventing an operator from being annoyed to wait for display requiredfor diagnosis.

[0026] (15) This display apparatus of X-ray projection images formedical use is to achieve the display method in the above-mentioned (5).Therefore, it has the same effect as (5). Further, the X-ray projectionimages which have been once stored are read out to be displayed so thatthe X-ray projection images aren't required to be produced each time.Therefore, the X-ray projection images can be promptly displayed, thuspreventing an operator from being annoyed to wait for display requiredfor diagnosis.

[0027] (16) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display method (6).Therefore, it has the same effect as (6).

[0028] (17) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display method (7).Therefore, it has the same effect as (7).

[0029] (18) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display method (8).Therefore, it has the same effect as (8).

[0030] (19) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display method (9).Therefore, it has the same effect as (9).

[0031] (20) This display apparatus of X-ray projection images formedical use is to achieve the display method in the above-mentioned(10). Therefore, it has the same effect as (10).

[0032] (21) This display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display method (11).Therefore, it has the same effect as (11).

[0033] (22) In this X-ray CT apparatus for medical use, an imageconstruction means having an X-ray generator and an X-ray detector iscombined with the display method (15). It achieves the display method(5) and has the effect of (5) as a medical X-ray CT apparatus.

[0034] (23) This X-ray CT apparatus for medical use achieves the displaymethod in (6) and has the effect of (6).

[0035] (24) This X-ray CT apparatus for medical use is for obtainingX-ray panoramic images, achieves the display methods in (10) and (11)and has the effects of (10) and (11).

[0036] (25) According to this X-ray CT apparatus for medical use, arotation center of a rotary arm is moved, not being fixed during X-rayradiation. For example if a rotation center of radiated X-rays is movedalong an envelope curve, X-rays can be radiated on a dental arch from adirection substantially perpendicular to a tooth, thereby achievingvaluable images for diagnosis without obstacle shades.

[0037] (26) According to this X-ray CT apparatus for medical use,comparing with the apparatus (25), a rotation center of a rotary arm isfixed, an object is gradually moved during X-ray radiation, thus arotation center of the radiated X-rays is relatively moved. In additionto the same effect as (25), more precise X-ray radiation can beaccomplished because the rotation center of the rotary arum isn't moved.More accurate three dimensional X-ray absorption coefficient data can beobtained from thus obtained transmitted data and as the result moreaccurate X-ray projection image can be also obtained.

[0038] (27) According to this X-ray CT apparatus for medical use, X-raysare radiated while varying a rotational speed of a rotary arm.Therefore, density compensation can be executed according to theradiated tooth, thus obtaining better X-ray projection images.

[0039] (28) According to this X-ray CT apparatus for medical use, likethe apparatus (27), density compensation can be executed according tothe radiated tooth, thus obtaining better X-ray projection images.

[0040] (29) According to this X-ray CT apparatus for medical use,inclination of a chair for holding an object is adjusted in such amanner that an X-ray radiating direction becomes, for example,perpendicular to a rising direction of a tooth, thereby obtainingtransmitted images without inclining the rising direction of a tooth.Accordingly, better X-ray projection images reflecting the risingdirection of a tooth can be obtained.

[0041] (30) This recording medium saves a program to achieve the displaymethod (1). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (1) isachieved and the effect of (1) is brought out.

[0042] (31) This recording medium saves a program to achieve the displaymethod (2). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (2) isachieved and the effect of (2) is brought out.

[0043] (32) This recording medium saves a program to achieve the displaymethod (3). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (3) isachieved and the effect of (3) is brought out.

[0044] (33) This recording medium saves a program to achieve the displaymethod (4). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (4) isachieved and the effect of (4) is brought out.

[0045] (34) This recording medium saves a program to achieve the displaymethod (5). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (5) isachieved and the effect of (5) is brought out.

[0046] (35) This recording medium saves a program to achieve the displaymethod (6). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (6) isachieved and the effect of (6) is brought out.

[0047] (36) This recording medium saves a program to achieve the displaymethod (7). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (7) isachieved and the effect of (7) is brought out.

[0048] (37) This recording medium saves a program to achieve the displaymethod (8). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (8) isachieved and the effect of (8) is brought out.

[0049] (38) This recording medium saves a program to achieve the displaymethod (9). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (9) isachieved and the effect of (9) is brought out.

[0050] (39) This recording medium saves a program to achieve the displaymethod (10). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (10)is achieved and the effect of (10) is brought out.

[0051] (40) This recording medium saves a program to achieve the displaymethod (11). When the medium reads the program and is attached to anapparatus capable of carrying out the program, the display method (11)is achieved and the effect of (11) is brought out.

BRIEF DESCRIPTION OF DRAWINGS

[0052]FIG. 1a is a conceptual diagram explaining a procedure of adisplay method of X-ray projection images for medical use according tothe present invention, FIGS. 1b, 1 c and 1 d show an example of X-rayprojection images of a tooth shown according to the method.

[0053]FIG. 2 is an example of displays shown by a display method ofX-ray projection images for medical use according to the presentinvention.

[0054]FIG. 3 is an example of X-ray projection images shown by a displaymethod of X-ray projection images for medical use according to thepresent invention.

[0055]FIG. 4 is other example of X-ray projection images shown by adisplay method of X-ray projection images for medical use according tothe present invention.

[0056]FIG. 5 is other example of X-ray projection images shown by adisplay method of X-ray projection images for medical use according tothe present invention.

[0057]FIG. 6 is other example of X-ray projection images shown by adisplay method of X-ray projection images for medical use according tothe present invention.

[0058]FIG. 7 is other example of X-ray projection images shown by adisplay method of X-ray projection images for medical use according tothe present invention.

[0059]FIGS. 8a, 8 b and 8 c show an example of X-ray panoramic imagesshown by a display method of X-ray projection images for medical useaccording to the present invention.

[0060]FIG. 9 is a flow chart showing one example of procedures of adisplay method of X-ray projection images for medical use according tothe present invention.

[0061]FIG. 10 is a conceptual view explaining radiography wherein alocal X-ray CT method used in a display method of X-ray projectionimages for medical use according to the present invention is applied topanoramic images.

[0062]FIG. 11 is a conceptual diagram explaining one example of settingmethods of a rotation center of a rotary arm when a local X-ray CTmethod is applied to panoramic images.

[0063]FIG. 12 is a conceptual diagram explaining other example ofsetting methods of a rotation center of a rotary arm when a local X-rayCT method is applied to panoramic images.

[0064]FIGS. 13a and 13 b are conceptual diagrams showing one example ofX-ray radiation method of X-ray CT used by an X-ray CT apparatus formedical use according to the present invention. FIGS. 13c and 13 d showone example of X-ray projection images obtained by the method. FIGS. 13eand 13 f show one example of X-ray projection images obtained by a priorart.

[0065] In FIG. 14 the X-ray radiation method in FIG. 13 is applied toother teeth. FIG. 14a is a conceptual diagram when the teeth are seenfrom a direction of a general normal line, FIG. 14b shows thus obtainedX-ray projection image, FIG. 14c is a conceptual diagram when the teethare seen from a normal line into a rising direction of the teeth, andFIG. 14d shows thus obtained X-ray projection image.

[0066]FIGS. 15a and 15 b are conceptual diagrams showing other exampleof an X-ray radiation method in an X-ray CT used in an X-ray CTapparatus for medical use according to the present invention.

[0067]FIG. 16 shows a conceptual diagram showing other example of anX-ray radiation method in an X-ray CT used in an X-ray CT apparatus formedical use according to the present invention.

[0068]FIG. 17 shows a conceptual diagram showing other example of anX-ray radiation method in an X-ray CT used in an X-ray CT apparatus formedical use according to the present invention.

[0069]FIG. 18 shows a basic construction of one example of an X-ray CTapparatus according to the present invention.

[0070]FIG. 19 is a block diagram showing an image signal processingsystem of an X-ray CT apparatus according to the present invention.

[0071]FIG. 20 is an outline view showing one example of an X-ray CTapparatus of the present invention.

[0072]FIG. 21a is a detailed front view of the apparatus of FIG. 20 andFIG. 21b is its side view.

[0073]FIG. 22 shows a diagrammatic configuration of a direction settingmeans of a rotation axis and a setting means of an object supportingdirection provided for an X-ray CT apparatus according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0074]FIG. 1a is a conceptual diagram explaining a procedure of adisplay method of X-ray projection images for medical use according tothe present invention, FIGS. 1b, 1 c and 1 d show an example of X-rayprojection images of a tooth shown according to the method.

[0075]FIG. 1a shows an image wherein three dimensional X-ray absorptioncoefficient data obtained by an X-ray CT method in which an X-raygenerator 1 and an X-ray detector 2 are opposed and rotated around adental arch S, being an object to be examined, are seen from an imagelayer CP having a predetermined thickness in a direction perpendicularto a radiated plane obtained by rotation of X-ray radiation, namelyshows an image when three dimensional X-ray absorption coefficient dataare seen from an axial direction of a rotation center 3 a of X-rayradiation.

[0076] Circular area around the center of the image layer CP is calledas a virtual local region Q which is always locally radiated with X-raysin a radiography wherein a local X-ray CT method is applied to panoramicimages as mentioned hereinafter. When an object is a dental arch S, theregion Q is generally selected around a median line inside of the dentalarch. Here in this specification, the virtual local region Q and theareas around four corners of the figure are eliminated from the area tocalculate three dimensional X-ray absorption coefficient data to be leftas more density parts.

[0077] According to the display method of X-ray projection images formedical use of the present invention, a projection interested area PA toconstruct the X-ray projection images of the object, namely a dentalarch S in this specification, is set under the following procedures.

[0078] <Setting of Projection Interested Area PA>

[0079] 1. As shown in FIG. 1a, the image of the image layer CP isdisplayed.

[0080] 2. Border lines of the dental arch S and a jawbone Sg areappeared on the image layer CP, although unclear, in such a manner theirposition on a radiated plane can be specified. A dental arch area toextract three dimensional X-ray absorption coefficient data is definedaiming at the border lines so as to include the dental arch S and itssupporting alveolar therearound.

[0081] 3. Each specific point on the border lines of the projectioninterested area PA in the figure is specified, for example, on the imageof the image layer CP by means of a pointing device such as a mouse (notshown) and the points are combined with a line so as to be compensated,thereby achieving setting. Dental arch area pattern calculated from thepast statistical data is applied per the kinds of the object, forexample age and sex, thereby achieving setting.

[0082] 4. In this case, as shown in the figure, such parts as a neckbone being an obstacle shade Ob in the projection interested area PA aredesigned to be eliminated.

[0083] Thus the projection interested area PA is set in advance, threedimensional X-ray absorption coefficient data only in this area PA areused for projection, and X-ray projection images without obstacle shadesOb can be obtained.

[0084] Then, the projection interested area PA is divided into pluralprojection interested layers Pal and Pa2 which are adjacent each otherin a direction of an X-ray radiating direction RD. Here the dental archarea is set as a projection interested area PA and the area PA isdivided into two parts Pal which is a cheek side and Pa2 which is atongue side by a center line SC of the dental arch. The setting methodof the center line SC of the dental arch is the same as that of theprojection interested area PA. If a shape of the projection interestedarea PA is once determined, sometimes all required is to specify thethickness of the produced projection interested layer in order to dividethe area PA into plural projection interested layers adjacent each otherin the X-ray radiating direction RD at each area PA.

[0085] As shown in the figure, for obtaining the X-ray projection imageof a tooth S7 of the dental arch S, a projection plane TP intersectingthe X-ray radiating direction RD from which the tooth S7 is radiated isset. The projection plane TP is thus set and only effective threedimensional X-ray absorption coefficient data aligning along the X-rayradiating direction RD are projected on the projection plane TP, thusobtaining clear X-ray projection images.

[0086] After the projection plane TP is set, the three dimensional X-rayabsorption coefficient data in the projection interested area PA areprojected on the projection plane TP only from the projection plateside, in this case from the opposite side of the X-ray radiatingdirection RD against the projection plane TP. Then the X-ray projectionimage TI of the tooth S7 as shown in FIG. 1c can be obtained.

[0087] Although it can be understood there exist three dental roots onthe X-ray projection image TI of the tooth S7, it is difficult todetermine whether those roots exist in a cheek side or in a tongue side.In this case, the X-ray projection image using the projection interestedlayers Pa1 and Pa2 is helpful.

[0088] When the three dimensional X-ray absorption coefficient data ofthe projection interested layer Pa1 at cheek side is projected on theprojection plane TP, the X-ray projection image TI of the tooth S7 shownin FIG. 1b can be obtained. When the three dimensional X-ray absorptioncoefficient data of the projection interested layer Pa2 at tongue sideis projected on the projection plane TP, the X-ray projection image TIof the tooth S7 shown in FIG. 1d can be obtained. From the X-rayprojection images TI shown in FIGS. 1b and 1 d, it is understood thatthere is one root at cheek side and other two roots are at tongue side.

[0089] Exemplifying a dental arch as mentioned above, what exists inwhich image layer can be understood as an X-ray projection image bydividing the projection interested area PA into plural projectioninterested layers Pa, that is for example a dental root exists where inthe cheek side to the tongue side.

[0090] More detailed positional detection is possible by appropriatelyselecting a projection interested area PA at first. Further it is alsopossible by dividing the area into a larger number of projectioninterested layers.

[0091]FIG. 2 is an example of displays shown by a display method ofX-ray projection images for medical use according to the presentinvention.

[0092] This figure is a display arranging showing in array theprojection interested area PA explained in FIG. 1, an image of the imagelayer CP on which a projection interested area PA, and projectioninterested layers Pa1 and Pa2 are set, and an X-ray projection images TIof tongue side, entire area and cheek side produced for each part of thedental arch according to the method mentioned in FIG. 1.

[0093] The X-ray projection image TI is obtained in such a manner that arotation center Ta of the projection plane TP is aligned with a rotationcenter 3 a of X-ray radiation to be fixed, a tooth of which X-rayprojection image is required is moved in sequence, and the projectionplane TP is correspondingly rotated. When the center Ta of theprojection plane TP is fixed according to radiography conditions,radiation conditions and radiography conditions are conformed so thatclear X-ray projection images can be obtained and further projectionprocedures are simplified.

[0094] Partial tooth images of the entire dental arch S are arranged ona display in array and they can be seen contrasting the tongue side, theentire area and the cheek side, therefore treatment parts are easilyspecified so as to be useful for medical care. The projection interestedarea and the projection interested layer are shown at one time so thattheir corresponding relation can be easily understood. Further, if namesof the selected projection interested area and the projection interestedlayer are also shown beside the X-ray projection images, more accuratediagnosis is possible.

[0095]FIG. 3-FIG. 7 show an example of X-ray projection images shown bya display method of X-ray projection images for medical use according tothe present invention. They are X-ray projection images constructed forbroader area than the image shown in FIG. 2. FIG. 3 is an X-rayprojection image TI at a cheek side when a dental arch is seen fromleft, FIG. 4 is an X-ray projection image TI at a tongue side seen fromthe same direction as FIG. 3, FIG. 5 is an X-ray projection image TI ofthe entire area seen from the same direction as FIG. 3, FIG. 6 is anX-ray projection image TI of the entire area when a dental arch is seenfrom the front and FIG. 7 is an X-ray projection image TI seen fromright.

[0096] From the X-ray projection images TI in FIG. 3, FIG. 4 and FIG. 5,the dental arch S seen from a specific direction can be understood,namely a dental arch S is perspectively displayed as a normal view.Comparing with a panoramic image shown in FIG. 8, those images TI areeasily understood by intuition and available for determination materialfor diagnosis.

[0097] Those images may be shown on a display respectively or may beshown in array like FIG. 2 such as cheek side, tongue side and entirearea per each part. If they are arranged in array, each image is easilycompared and the position of dental root from the cheek side to thetongue side is obtained.

[0098] From the X-ray projection images TI in FIG. 5, FIG. 6, and FIG.7, X-ray projection images are shown by rotating the projectingdirection to the dental arch, such as left, front and right. When theimage of the dental arch being an object is displayed in rotation bysequentially and continuously showing plural X-ray projection images,even if a display area is limited, X-ray projection images seen fromdifferent directions can be continuously displayed, thus enablingselection of an image required for diagnosis and achieving convenience.

[0099] Specifically, in case of a dental arch, about 512 X-rayprojection images taken from different projection angles aresequentially and continuously displayed at intervals from 1/30 sec. to1/15 sec., thereby displaying in rotation a maxillo facial area.

[0100] Combining a display in rotation and a display in array, X-rayprojection images of cheek side, tongue side and entire area aredisplayed in array and each X-ray projection image is displayed inrotation by sequentially changing the projecting direction.

[0101]FIGS. 8a, 8 b and 8 c show an example of X-ray panoramic imagesshown by a display method of X-ray projection images for medical useaccording to the present invention.

[0102] Generally in case of X-ray panoramic images, a projection planeto obtain panoramic images isn't flat like the X-ray projection image inFIG. 1 but is curved and its projecting direction is as a ruleperpendicular to a part of each curved projection plane. Therefore, themethod of the present invention isn't applied as it is.

[0103] However, application of a projection interested area and aprojection interested layer is useful for constructing X-ray panoramicimages. The three dimensional X-ray absorption coefficient data to beprojected on the curved projection plane in order to obtain X-raypanoramic images can be limited to those in the projection interestedarea or the projection interested layer.

[0104]FIGS. 8a, 8 b and 8 c show X-ray panoramic images V obtained bysetting the projection interested area PA and the projection interestedlayers Pa1 and Pa2 as shown in FIG. 1 against the dental arch S. FIG. 8ais a panoramic image V at cheek side, FIG. 8b shows a panoramic image Vof the entire area and FIG. 8c shows a panoramic image V at tongue side.If each name of the setting area or setting layer such as cheek side,entire area and tongue side is described beside the image, mistakes areavoided and accurate diagnosis can be executed.

[0105] As mentioned above, if the projection interested area andprojection interested layer are applied to X-ray panoramic images, animage which has little obstacle shade and can show the position of adental root from cheek side to tongue side can be obtained.

[0106]FIG. 9 is a flow chart showing one example of procedures of adisplay method of X-ray projection images for medical use according tothe present invention. The display procedure of the present inventionwill be detailed hereinafter. The procedure for obtaining threedimensional X-ray absorption coefficient data in order to apply thedisplay method of the present invention will be explained here, however,the object of the present invention is to provide a display method andthe invention isn't limited to a production method of three dimensionalX-ray absorption coefficient data.

[0107] At first three dimensional X-ray absorption coefficient data arerequired to be obtained by radiating X-rays on an object to be examined.To this end, an imaging mode of X-ray CT is selected (S1). The reason isthat this invention applies as a pre-stage a local X-ray CT method,namely a radiography in which the effects of the display method of thepresent invention are interactively achieved, to obtain X-ray panoramicimages. In such a case, an imaging mode is required to be selectedwhether a general local CT mode or a CT mode for panoramic images. Inthis case, a panoramic CT is selected.

[0108] Next, the object sits on a chair for picturing shown as thereference numeral 4 in FIG. 20. His head is fixed with a holding means 4a shown in FIG. 20 to set X, Y and Z positions (S2), a rotation center 3a of a rotary arm 3 in FIG. 20, namely a rotation center of X-rayradiation, is set to be a center Qa of a virtual local region Q in caseof panoramic CT mode (S3).

[0109] Then conical X-ray beams are locally radiated according to aprojection mode while the rotary arm is turned in a fixed angle areacorresponding to a projection mode (S4), an image processing includingbackprojection is executed based on the obtained X-ray transmitted dataaccording to a projection mode to obtain three dimensional X-rayabsorption coefficient data (S5), and the data are stored (S6).

[0110] Thus, after obtaining three dimensional X-ray absorptioncoefficient data, a projection interested area PA is set as mentionedabove for the obtained three dimensional X-ray absorption coefficientdata (S11), a projection interested layer Pa is selected if necessary(S12), a projection plane TP from an X-ray radiating direction on apoint where projection is desired is determined and a projection figureis produced by projecting three dimensional X-ray absorption coefficientdata on the projection interested area PA or the projection interestedlayer Pa depending on the selection (S13), then the figure is stored(S14). These procedures are repeated for a desired range in theprojection interested area PA (S15).

[0111] After producing and storing the projection figure, a displaymethod is selected from a display in array, a rotary display and theircombination (S16), the figures are shown on a display means according tothe selected display method (S17), and the displayed images are storedif necessary (S18). These procedures are repeated in a required area andfinished (S19).

[0112] According to the above-mentioned procedures, advantages of thelocal X-ray CT method and apparatus and three dimensional X-rayabsorption coefficient data are effectively used, thereby enablingdisplay of X-ray projection images which has little obstacle shades, iseasily understandable by intuition for medical diagnosis, and can showthe position of dental root from a cheek side to a tongue side.

[0113]FIG. 10 is a conceptual view explaining a radiography methodwherein a local X-ray CT method used in a display method of X-rayprojection images for medical use according to the present invention isapplied to panoramic images. The reference numerals already explainedhave the same numerals and their explanations are omitted hereinafter.

[0114] In FIG. 10, a rotary arm 3 having an X-ray generator 1 and a twodimensional X-ray image sensor 2, which is an X-ray detector, at bothends thereof is turned at a constant velocity so as to keep the widthbetween the rotation center 3 a of the rotary arm 3 and a conical X-raybeam 1 a so as to form a virtual local region Q. The X-ray generator 1radiates a conical X-ray beam 1 a with a fixed width in a scanningdirection accompanying with the movement of the rotary arm 3 andsequentially produces X-ray transmitted images of a dental arch S on thetwo dimensional X-ray image sensor 2 by the conical X-ray beam 1 a. Onlya partial X-ray transmitted images produced by an ortho-conical X-raybeams 1 b substantially orthogonal to the dental arch S among the beambundles of the conical X-ray beam 1 a is extracted for the X-raytransmitted images sequentially produced on the two dimensional X-rayimage sensor 2. The extracted partial X-ray transmitted images arearithmetically processed to obtain three dimensional X-ray absorptioncoefficient data of the dental arch area including the dental arch S.

[0115] Thus applying a basic local X-ray CT method wherein the rotaryarm 3 is turned with its center 3 a fixed during radiation and a conicalX-ray beam 1 a with a predetermined width is locally radiated, threedimensional X-ray absorption coefficient data for X-ray panoramic imagescan be produced. Further, the X-ray exposure duration is short and theexposed dose amount is reduced into 1/50 of the prior CT method, inaddition a sufficient three dimensional X-ray absorption coefficientdata can be obtained like the prior art.

[0116]FIG. 11 is a conceptual diagram explaining one example of settingmethods of a rotation center of a rotary arm when a local X-ray CTmethod is applied for panoramic images.

[0117] According to a local X-ray CT method, the rotation center 3 a ofthe rotary arm 3 is aligned with the center of a virtual local region onan axis of symmetry Lo at the center of the dental arch area SA, therotary arm is turned at a constant velocity or a variable speed in arotary angle area according to radiography conditions, and a conicalX-ray beam with a fixed width is locally radiated, thus obtained X-raytransmitted images of the dental arch area SA.

[0118] Generally in case of prior film-type panoramic imaging, it isrequired to execute radiography while an X-ray beam bundle is moved bytransferring the rotation center of the rotary arm in such a manner thatX-rays become orthogonal to the dental arch for each tooth in the curvedsectional area SA. In FIG. 11, such X-ray beam bundle is shown with thereference numeral L. When X-ray beam bundles L . . . substantiallyorthogonal to each tooth are shown for the dental arch area SA, anenvelope line La of each X-ray beam bundle L . . . is produced. Assumingan incircle G which touches internally the envelope line La, all theX-ray beams for the dental arch area SA passes through the incircle G.

[0119] In this local X-ray CT, a center Ga of the incircle G is alignedwith the center 3 a of the rotary arm 3 and radiation is executed byturning the rotary arm without moving the rotation center 3 a thereof.Conical X-ray beam 1 a with a fixed width so as to include the incircleG is locally radiated on the object from its surround. The conical X-raybeam 1 a always includes X-ray beam bundles substantially orthogonal tothe dental arch area SA (called as an ortho-conical X-ray beamhereinafter).

[0120] In this example, the incircle G becomes a virtual local region asshown in FIG. 10, which is also referred to the reference numeral Q. TheX-ray beam bundles orthogonal to each tooth are the ortho-conical X-raybeams as mentioned above, which are shown as the reference numeral 1 b.

[0121] When the X-ray conical beam 1 a is locally radiated so as to formthe virtual local region Q, partial X-ray transmitted images obtained bythe ortho-conical X-ray beams lb substantially orthogonal to the dentalarch area SA are extracted from the X-ray transmitted images of thedental arch area SA sequentially produced on the two dimensional X-rayimage sensor 2 and are arithmetically processed to obtain the threedimensional X-ray absorption coefficient data of the dental arch areaSA.

[0122] A projection method wherein the local X-ray CT method used in theX-ray CT apparatus of the present invention is applied to panoramicimages is based on the above-mentioned concept. The position of therotation center 3 a of the rotary arm 3 and the width of the conicalX-ray beam 1 a, namely the position and size of the specific region Q,are appropriately set according to the images to be produced finally. Insummary, ortho-conical X-ray beams according to the images are necessaryto be included in conical X-ray beams.

[0123] For example, the center 3 a of the rotation center 3 which is setat the same time of radiation and the width of conical X-ray beam, thatis the virtual local region Q, aren't limited to the above-mentionedincircle G internally touching the envelope line La. It may be anincircle G′ or G″ shown in FIG. 11. If such a circle is defined as theregion Q, the center of the circle is always on the axis of symmetry Loinside of the dental arch S. The position of the center 3 a of therotary arm 3 when the region Q is the incircle G′ or G″ is shown as thereference numerals 3 a′ or 3 a″.

[0124] X-ray panoramic images are not limited to ortho-radial X-raypanoramic images in which X-ray beam bundles are substantiallyorthogonal to the tooth. There are standard X-ray panoramic images,X-ray panoramic images of a jawbone and X-ray panoramic images on eitherone side of right or left. In such images, ortho-conical X-ray beamsaren't always orthogonal to the dental arch S. If X-ray panoramic imagesby such a radiography method are produced, the position of the center 3a of the rotary arm 3 on the axis of symmetry Lo of the dental arch Sand the width of conical X-ray beam 1 a, namely the local region Q, arerequired to be determined so as to include all the ortho-conical X-raybeams 1 b. Such examples are the above-mentioned incircles G′ and G″.For producing X-ray panoramic images on either side of right or left,the center 3 a of the rotary arm 3 may be inside of the dental arch areaSA or may not be on the axis of symmetry Lo.

[0125] The local region Q to produce the X-ray panoramic images isdetermined corresponding to the X-ray panoramic images to be produced.Smaller area is better considering the reduction of the X-ray exposureamount.

[0126] As understood from FIG. 11, the rotary arm 3 is required to beturned over 180 degrees, however it isn't required to be turned 360degrees. Namely radiation is executed by turning the rotary arm 3 from180 degree to 360 degree. If the angle is small, the X-ray exposed dosecan be reduced accordingly.

[0127] In this invention an improved radiography method when the localX-ray CT method is applied to panoramic images is also proposed suchthat the rotation center 3 a is moved by the minute during X-rayradiation without fixing the center 3 a. For example, the center 3 a maybe moved along an envelope line La like a film-type radiography.

[0128] In such a case, while keeping the effect of reduction of thelocal X-ray exposed amount and the effect of reduction of imaging timewhich are characteristic of the local X-ray CT, only more narrowortho-conical X-ray beams are radiated although a movement control ofthe rotary arm 3 becomes complicated. Further, a radiated tooth isdisposed near the rotation center so that transmitted images with aconstant size can be always radiated on the X-ray detector 2. Therefore,the detector area is effectively used, images with constant resolutionand accuracy can be obtained. This method is also generally used otherthan panoramic images.

[0129] When the rotation center 3 a is moved during X-ray radiation, therotation center Ta of the projection plane TP is correspondingly movedas explained referring to FIG. 1, then the projection conditions conformwith the radiation conditions to obtain clearer X-ray projection images.

[0130] Further, if an X-ray tube voltage and/or an X-ray tube currentare changed during X-ray radiation, or if X-rays are radiated whilevarying the rotation speed of the rotary arm 3, density compensationdepending on the radiated tooth can be executed, thus obtaining betterX-ray projection images.

[0131]FIG. 12 is a conceptual diagram explaining other example ofsetting methods of a rotation center of a rotary arm when a local X-rayCT method is applied to panoramic images.

[0132] In this embodiment, there exists metal M such as crown at pluralteeth. If X-rays are radiated from the conical X-ray beam 1 a(1) to 1a(2) while fixing the center 3 a of the conical X-ray beam 1 a, X-raysare radiated on both metals M of a forward teeth and of a rear teeth atthe conical X-ray beam 1 a(2). In such a case a metal artifact isgenerated so that correct X-ray transmitted images can't be obtained andhence correct three dimensional X-ray absorption coefficient data can'tbe obtained.

[0133] In order to avoid such simultaneous radiation on plural metals M,X-rays are radiated by moving the rotation center 3 a in such a mannerthat the conical X-ray beam la is positioned like 1 a(3) after 1 a(1).

[0134] The effect of moving the rotation center 3 a is to avoid themetal artifact as mentioned above, thereby obtaining better threedimensional X-ray absorption coefficient data and better X-rayprojection images.

[0135]FIGS. 13a and 13 b are conceptual diagrams showing one example ofX-ray radiation method in an X-ray CT used for an X-ray CT apparatus formedical use according to the present invention. FIGS. 13c and 13 d showone example of X-ray projection images obtained by the method. FIGS. 13eand 13 f show one example of X-ray projection images obtained by a priorart.

[0136]FIG. 13a shows the dental arch area SA in a form of a verticalsection of a front jaw. In the figure, an upper front tooth S1U and alower front tooth S1L are shown. FIGS. 13a, 13 c and 13 e show the upperfront tooth S1U and FIGS. 13b, 13 d and 13 f show the lower front toothS1L.

[0137] In this method, X-rays are radiated on a normal line γ against arising direction T1U of the upper front tooth S1U. The projection planeTP perpendicular to the normal line γ is determined and threedimensional X-ray absorption coefficient data on the dental arch area SAwhich is also a projection interested area are projected on the planeTP, thus obtaining X-ray projection images.

[0138] This method is applied to the lower front tooth S1L in the samemanner. The reference numeral T1L in the figure shows a rising directionof the lower front tooth S1L. The length of the upper front tooth S1Uand the lower front tooth S1L in a direction of a panoramic sectionalarea SB of the dental arch S is shown as WU and WL respectively, and thelength perpendicular to the nominal line γ is shown as VU and VLrespectively. The reference numerals H3 and H4 show a foreign materialhidden in the upper front tooth S1U and in the lower front tooth S1L.

[0139]FIG. 13c is a partial view of the upper front tooth S1U of theproduced X-ray projection image TI and FIG. 13d is a partial view of thelower front tooth S1L. FIGS. 13e and 13 f show partial views of theX-ray projection image TI projected on a projection plane TP including acentral perpendicular line SB of the dental arch area SA regardless arising direction.

[0140] Comparing those figures, it is understood that the length VU andVL which is an actual entire length of the upper front tooth S1U and thelower front tooth S1L and an accurate position of the foreign materialH3 and H4 for the length VU and VL are obtained by FIGS. 13c and 13 d.The lengths WU and WL obtained by FIGS. 13e and 13 f were varied by theangle of a rising direction T1U and T1L of the upper front tooth S1U andthe lower front tooth S1L, therefore accurate length were not obtained.

[0141] According to this method, the X-ray projection images in whicheach tooth is clearly appeared, not the X-ray projection images parallelto the central perpendicular line SB of the dental arch SA, can beobtained.

[0142] The normal line γ is changed as the rising direction is variedaccording to the tooth and is determined corresponding to each tooth.

[0143] In FIG. 14 the X-ray radiation method in FIG. 13 is applied toother teeth. FIG. 14a is a conceptual diagram when the teeth are seenfrom a direction of a general normal line, FIG. 14b shows thus obtainedX-ray projection image, FIG. 14c is a conceptual diagram when the teethare seen from a normal line into a rising direction of the teeth, andFIG. 14d shows thus obtained X-ray projection image.

[0144] Comparing each figure in FIG. 14, X-rays are radiated in such amanner that an upper X-ray radiating direction, a projecting directionγU, and a lower X-ray radiating direction, a projecting direction γLbecome a nominal line direction into a rising direction of the uppertooth SU and the lower tooth SL respectively. X-ray projection imagesare produced based on thus obtained three dimensional X-ray absorptioncoefficient data, the length of each tooth is shown in approximately thesame as an actual size and if the upper tooth and the lower tooth areoverlapped, the overlapped part is eliminated on the image, achievingconvenience for diagnosis. Otherwise, a radiography is executed from adirection orthogonal to a rotation axis of X-ray radiation and the X-rayprojection image is shown by setting a projection plane parallel to aninclination of the tooth only for display.

[0145]FIGS. 15a and 15 b are conceptual diagrams showing other exampleof X-ray radiation method in an X-ray CT method used for an X-ray CTapparatus for medical use according to the present invention.

[0146] As seen from the figures, if a rotation center 3 a of the rotaryarm 3, namely an axial direction of the rotation axis, is inclined forthe object O like FIG. 15a, or if the object O is inclined for the axialdirection of the rotation axis 3 a of the rotary arm 3 which is avertical direction, affect of obstacle shadows such as a jawbone iseliminated. In case of FIG. 15b, the object O which doesn't rotate isinclined so that the construction of an object holding means (describedlater) which is an inclination means is simplified.

[0147] Thus the radiating direction is inclined, for projecting theobtained three dimensional X-ray absorption coefficient data, theprojecting direction is aligned with the inclined direction, obtainingclearer X-ray projection images.

[0148] The flat plane perpendicular to the rotation center 3 a andproduced by radiating and rotating conical X-ray beams 1 a as a centralaxis of the rotation center 3 a is called as a radiation plane.

[0149]FIG. 16 shows a conceptual diagram showing other example of X-rayradiation method in an X-ray CT used in an X-ray CT apparatus formedical use according to the present invention.

[0150] As a method for inclining a radiating direction, other thaninclining the axial direction of the rotation center 3 a and incliningthe object, the X-ray generator 1 and the X-ray detector 2 are inclinedagainst the rotary arm 3 as shown in this figure. The same effect asmentioned above can be achieved.

[0151]FIG. 17 shows a conceptual diagram showing other example of X-rayradiation method in an X-ray CT used in an X-ray CT apparatus formedical use according to the present invention. In this method arotation axis of a rotary arm executes precession.

[0152] As seen from FIG. 17 while the rotation axis, the rotation center3 a of the rotary arm 3 executes precession, namely a grinding operationin such a manner that the center 3 a is rotated around a specific pointagainst a specific rotation axis 3 b keeping a predetermined angle intoa direction shown as an arrow, conical X-ray beams 1 a are radiated byturning the rotary arm 3. Comparing with a vertical radiation or with acase a rotation axis is simply inclined, alternatives of a projectionmethod of X-ray conical beams 1 a so as to avoid obstacle shades areincreased so that the method can more suitably correspond to severalkinds of obstacle shades.

[0153] In case of X-ray panoramic radiography, not a normal CT, X-raysare radiated with about 5 degrees of angle of elevation in order toalleviate the obstacle shades. Executing the same method for CT, X-raytransmitted images with less obstacle shades can be obtained and threedimensional X-ray absorption coefficient data with less obstacle shadescan be obtained by backprojection based on the X-ray transmitted images.

[0154] Also in this case, when the projecting direction is aligned withthe radiating direction executing precession, better X-ray projectionimages can be obtained.

[0155]FIG. 18 shows a basic construction of one example of an X-ray CTapparatus according to the present invention.

[0156] X-ray CT apparatus 20 in FIG. 18 is provided with an X-rayimaging means A, an X-ray beam width restriction means B, a rotary armdriving control means C, an arithmetic processing means D, a displaymeans E, an object holding means 4, a main frame 10, an operationconsole 11 and an operation panel 12.

[0157] The X-ray imaging means A has a rotary arm 3 from which an X-raygenerator 1 and a two dimensional image sensor 2 opposing each other aresuspended.

[0158] The X-ray generator 1 has an X-ray beam width restriction means Bwith a radiation control slit 8 and an X-ray beam controller 8 b in sucha manner that X-ray beams radiated from an X-ray tube are controlled bythe X-ray beam width restriction means B so as to emit conical X-raybeams 1 a or ortho-conical X-ray beams 1 b with a desired width.

[0159] The two dimensional X-ray image sensor 2 is a well known type inwhich an X-ray image intensifier II, in short X-ray II, and a CCD cameraare combined. In this sensor, the X-rays run into a scintillator layerprovided on the surface of the X-ray II is converted into a visiblelight and the visible light is converted into electron to beelectrically intensified by a photoelectric converter and the electronis converted to a visible light by a fluorescent material to be picturedby a two dimensionally arranged CCD camera (solid-state image sensingdevice) through a lens.

[0160] Otherwise, as an image sensor, a two dimensional X-ray imagesensor such as a cadmium telluride detector and a MOS sensor, and a wellknown two dimensional X-ray image sensor such as a CCD image sensorwhich is a combination of a scintillator, a glass fiber, and the CCD maybe used.

[0161] An XY table 31, an elevation control motor 32, a rotation controlmotor 33 are provided for the rotary arm 3. The rotation center 3 a isadjustable in an X-Y direction by controlling an X-axis control motor 31a and a Y-axis control motor 31 b and is adjustable vertically bydriving the elevation control motor 32. Further for imaging, therotation control motor 33 is driven at a constant velocity so as to turnthe rotary arm 3 about the object O. The elevation control motor 32constitutes a vertical arm position control means of the rotary arm 3.

[0162] The center 3 a of the rotary arm 3, namely the rotation axis, isprovided vertically and the rotary arm 3 is turned horizontally so thatconical X-ray beams la are locally and horizontally radiated, thusachieving a vertically constructed apparatus requiring smallinstallation area.

[0163] The rotation control motor 33 comprises a rotary drive means ofthe rotary arm 3, uses a motor such as a servo motor which can controlits rotational speed and rotational position freely, and is directly andaxially attached to the rotation center 3 a of the rotary arm 3.

[0164] Accordingly, the rotary arm 3 can be turned at a constantvelocity or a variable velocity and its rotational position can be knownalong a time axis so that it is available for taking out X-raytransmitted images by the two-dimensional image sensor 2 in exact timingwithout runout and thus a local X-ray CT method used in the presentinvention can be effectively executed.

[0165] A hollow part 3 b is provided for the rotation center 3 a of therotary arm 3. It is required to make a hollow part for all the membersprovided on the rotation center 3 a in order to have such a hollow part3 b. For this purpose, a servo motor with a hollow axis can be used as arotation control motor 33.

[0166] The hollow part 3 b is provided to arrange connection wiresbetween the X-ray generator 1 and the two-dimensional X-ray image sensor2 suspended from the rotary arm 3 and the operation console 11 of themain frame 10.

[0167] The method for arranging the wire becomes a problem in order toprovide an electric wring for rotating members. If the connection wireis thus arranged through the rotation center 3 a of the rotary arm 3,affection caused by rotation such as twist can be minimized and apreferable effect such as a beautiful appearance can be obtained.

[0168] Rotary means C in this embodiment is comprised of a combinationof the position control means 31 such as an XY table, the elevationcontrol motor 32 and the rotation control motor 33, however the presentinvention isn't limited to such construction. As the most easiestconstruction, the center 3 a of the rotary arm 3 may be manuallyoperated by a handle so as to position appropriately.

[0169] The object holding means 4 is provided with a head holding means4 a, a chair 4 b having the head holding means 4 a at the top thereof,and an object positioning mechanism 41. The object positioning mechanismincludes a holding means control motor 41 a for moving the head holdingmeans 4 a vertically, a back tilt control motor for controlling thetilting of the back of the chair 4 b, a movable table (not shown) onwhich the chair 4 b is placed and which linearly moves in X-axisdirection by an X-axis control motor 41 c, in Y-axis direction by aY-axis control motor 41 d and in Z-axis direction by a Z-axis controlmotor 41 e respectively.

[0170] The X-axis, the Y-axis, and the Z-axis linearly moving table isconstructed with well known cross roller guides and a combination oftypical bearings and guides so as to be movable linearly in an accuratemanner. While a rack-and-pinion method, a ball screw system or a generalscrew axis is applied for the X-axis, Y-axis and Z-axis linearly movingtable by means of the motors 41 c-41 e as a driving source, anaccurately positioning method is desirable.

[0171] The object O sits on the chair 4 b and the position of the headholding means 4 a is adjusted. Then the head of the object O is securedand the center 3 a of the rotary arm 3 and the center Qa of the virtuallocal area Q in the object O are aligned by means of the objectpositioning mechanism 41. Further, the rotary arm 3 is turned duringX-ray radiation and also the center 3 a is simultaneously moved.Otherwise, the rotation center 3 a is fixed and alternatively the chair4 b is horizontally moved to move the object O.

[0172] Suitable positioning for imaging can be done while the objectsits on the chair, thereby achieving an apparatus which is gentle forthe object.

[0173] Arithmetic processing means D includes a processor operable athigh speed for image processing and analysis. A predetermined processingis executed after the X-ray transmission image produced on thetwo-dimensional image sensor 2 is preprocessed so that three-dimensionalX-ray absorption coefficient data in the object through which X-rays aretransmitted are calculated. Furthermore, computation such as projectionof the data on a projection plane is executed, then an X-ray projectionimage or a panoramic X-ray image is shown on the display means E and isstored in a required storage means as an image information.

[0174] The arithmetic processing means D may be executed sequentiallyduring radiation or may be executed if necessary after radiation.

[0175] The main frame 10 is a structure supporting the entire X-rayapparatus 20 and will be detailed hereinafter. The operation console 11controls the apparatus 20 entirely and executes several control commandsreceiving input from the operation panel 12. Input and operationrequired for setting the apparatus 20 are executed by the operationpanel 12.

[0176]FIG. 19 is a block diagram showing an image signal processingsystem of an X-ray CT apparatus according to the present invention.

[0177] This system includes the arithmetic processing means D as a mainpart, the X-ray generator 1, the two dimensional X-ray image sensor 2,the operation panel 12, the display means E, and the storage means F,all of which are connected with the arithmetic processing means D. Thearithmetic processing means D is provided with a control means Da, animage storage means Db, an A/D converter Dc, and a processor memory Dd.

[0178] Such an arithmetic processing means D is constructed with a microprocessor for image processing.

[0179] The X-ray transmitted image data received from the twodimensional X-ray image sensor (detector) 2 are converted into digitalsignals by the A/D converter Dc and the digitalized image data arestored in the image storage means Db. Plural image data in the imagestorage means Db are stored in the processor memory Dd. Calculation ofthree dimensional X-ray absorption coefficient data by backprojectioncorresponding to an selected projection mode and operation such asprojection of the data on a projection plane are executed for the storeddata to produce X-ray projection images and X-ray panoramic images.

[0180] These images are shown on the display means E and the threedimensional X-ray absorption coefficient data, the X-ray projectionimage data, the X-ray panoramic image data are stored in the storagemeans F if necessary. The obtained three dimensional X-ray absorptioncoefficient data may be stored in the image storage means Db associatedwith the original image data.

[0181] The display means E includes an interested area setting means Eafor setting a projection interested area and a projection interestedlayer, an interested layer selection means Eb for selecting a projectioninterested layer, a projection plane setting means Ec for setting aprojection plane, and an angle setting means in a Z-axis direction Edfor setting an angle of a projection plane in a Z-axis direction. Theangle setting means in a Z-axis direction Ed has a variable means Edafor variably setting an angle in stepless and a selection means Edb forselecting an angle from plural angles, by either of which the angle ofthe projection plane in a Z-axis direction can be set.

[0182] After the three dimensional X-ray absorption coefficient data arecalculated, on the display means E, a projection interested area and aprojection interested layer are set by the interested area setting meansEa, an interested layer is appropriately selected by the interestedlayer selection means Eb, and a projection plane is sequentially set bythe projection plane setting means Ec and the angle setting means Ed,producing an X-ray projection image. Thus produced X-ray projectionimages are displayed in array, in sequence or in continuous so that theobject is displayed in a rotational manner (in a manner that the objectis rotated).

[0183] Hard disc apparatus, a magneto-optic disc apparatus and a framememory are used for an image storage means Db and a storage means F.

[0184]FIG. 20 is an outline view showing one example of an X-ray CTapparatus of the present invention. FIG. 21a is a detailed front view ofthe apparatus of FIG. 20 and FIG. 21b is its side view.

[0185] The main frame 10 which is formed like a gate and is highly rigidstructure is designed to support the entire X-ray CT apparatus 20.

[0186] The main frame 10 has an arm 10 a for rotatably supporting therotary arm 3 suspending the X-ray generator 1 and the two-dimensionalX-ray imaging sensor 2 in opposed condition, a pair of lateral beams 10b securely supporting a base end of the arm 10 a, a pair of verticalbeams 10 c supporting the lateral beams 10 b, and a base 10 d on which apair of vertical beams 10 c are securely placed and which is a base ofthe entire apparatus 20.

[0187] A highly rigid steel material is used for the members of the mainframe 10 and braces and angular reinforcing members are appropriatelyused for resisting deformation so as not to vary the rotation center 3 aof the rotary arm 3 during rotation.

[0188] The main frame 10 is constructed not to cause rotary deflectionof the rotary arm 3, thereby it is applicable for the X-ray CT apparatuswhich requires no rotary deflection.

[0189] The operation panel 12 is arranged on the surface of the verticalbeam 10 c of the main frame 10 and where an operator easily uses itwhile standing. On the object positioning mechanism 41 as explainedreferring to FIG. 19, the chair 4 b of the object holding means 4 isplaced in such a manner that the chair 4 b is moved in an X-direction, aY-direction, or a Z-direction, namely in back and forth, vertically andin up and down respectively, and the back 4 ba of the chair 4 b istilted so as to hold the head of the object O being tilted.

[0190] The display means E constructed with a monitor such as CRT andthe arithmetic processing means D is comprised of a personal computer.Programs stored in the recording medium H such as a hard disc, a floppydisc, MO, CD, CDR and DVD are read out to be executed, and data are readin or read out.

[0191] A program to achieve the display methods of X-ray projectionimages for medical use according to the present invention can be storedin the recording medium H. If such a recording medium H is read in theX-ray CT apparatus having the above-mentioned processing means, severaldisplay methods of X-ray projection images for medical use according tothe present invention can be achieved.

[0192]FIG. 22 shows a diagrammatic configuration showing a directionsetting means of a rotation axis and an object tilting means providedfor an X-ray CT apparatus according to the present invention. Thisfigure is further comprised of a rotation axis direction setting means34 in addition to the basic construction shown in FIG. 20 and FIG. 21.

[0193] The rotation axis direction setting means 34 supports the XYtable 31 bracing the rotary arm 3 inclinably against the arm 10 a of themain frame 10 in such a manner that the center 3 a of the rotary arm 3,namely the axial direction of the rotation axis, is inclined into anoutlined arrow in the figure against a rising direction of the object O.The head holding means 4 a fixes the head of the object O and the headis tilted into an arrow with a solid line by tilting the back 4 ba ofthe chair 4 b having the head holding means 4 a.

[0194] This embodiment shows an X-ray CT apparatus supporting the objectsubstantially vertically. However, the apparatus may be constructed tosupport the object horizontally for a bed-ridden patient like a priorart, if large occupied area is allowed. Further, a rotary arm isconstructed as one arm type in the figures, however, a ring like arm maybe used like a prior CT apparatus and an X-ray generator and an X-raysensor may be provided at both ends so as to be opposed.

[0195] Industrial Applicability

[0196] 1. According to the display method of X-ray projection images formedical use, without using three dimensional X-ray absorptioncoefficient data obtained by an X-ray CT as it is for constructing anX-ray projection image, three dimensional X-ray absorption coefficientdata are extracted on an image layer having a predetermined thickness ina direction perpendicular to an X-ray radiation plane, namely on theimage wherein three dimensional X-ray absorption coefficient data areseen from a rotation axial direction of X-ray radiation, and aprojection interested area to construct an X-ray projection image isset.

[0197] A projection plane to be projected with an X-ray projection imageis set to be a flat surface intersecting a radiation plane, inparticular substantially perpendicular to an X-ray radiating direction.And an X-ray projection image is obtained by projecting threedimensional X-ray absorption coefficient data in the projectioninterested area with respect to the projection plane and the obtainedX-ray projection image is displayed. In such a manner, three dimensionalX-ray absorption coefficient data existing in a direction substantiallyalong the X-ray radiating direction so that clear images can beobtained. In addition, three dimensional X-ray absorption coefficientdata are projected on the flat projection plane. Therefore, exemplifyinga dental arch, images seen from a projecting direction perpendicular tothe projection plane can be obtained and they are easily comprehensiveimages by intuition for dental diagnosis.

[0198] Further, perspectively observable image like the dental arch,namely an object to be examined, is turned are obtained by sequentiallyobtaining and displaying X-ray projection images by rotating theprojecting direction, that is the projection plane, thus obtaininghighly comprehensive and highly advantageous images for diagnosis.

[0199] Therefore, when this display method is applied to the threedimensional X-ray absorption coefficient data obtained by a methodwherein the local X-ray CT method is applied to panoramic images, botheffects are multiplied so that several advantages of a local X-ray CTmethod and apparatus can be used.

[0200] 2. Comparing with the above-mentioned method 1, the displaymethod of X-ray projection images for medical use in other embodiment ischaracterized in that not only one X-ray projection image obtained bythe above-mentioned method is shown but also plural X-ray projectionimages can be shown in array. Therefore, in addition to the effects ofabove-mentioned, X-ray projection images in which an object such as adental arch is seen from different directions can be compared each otherand be compared in a list to select an image required for diagnosisbecause of such plural displaying, therefore this method is convenient.

[0201] 3. Comparing with the above-mentioned method 1, the displaymethod of X-ray projection images for medical use can show the object ina rotational manner by continuously showing the X-ray projection imagesobtained by the method 1 while changing the projecting directions.Therefore, in addition to the effects of 1, even if the display screenis limited, the X-ray projection images of an object such as a dentalarch can be continuously compared to select a necessary image fordiagnosis because of the continuous rotary display of the X-rayprojection images which are easily comprehended by intuition.Accordingly, this method is convenient.

[0202] 4. Comparing with the method 1, the display method of X-rayprojection images for medical is characterized in that the projectioninterested area is in advance divided into several projection interestedlayers neighboring each other considering an X-ray radiating direction,three dimensional X-ray absorption coefficient data in optional onelayer or the neighboring plural layers are used and thus obtained X-rayprojection images are selectively displayed. Therefore, in addition tothe effects of 1, if the projection interested area is a dental archarea, the projection interested layer is a layer neighboring each otherfrom a cheek to a tongue. Thus knowing the area or the layer used forconstructing the X-ray projection image of the dental root, the positionof the root where in a cheek to a tongue can be understood.

[0203] 5. Comparing with the method 1, the display method of X-rayprojection images for medical use combines the display in array in 2,the continuous display in 3 and the display method in 4 wherein theprojection interested area is divided into plural projection interestedlayers and the X-ray projection images obtained by using the threedimensional X-ray absorption coefficient data in the projectioninterested layer are selectively shown. Therefore, the effects of 2, 3and 4 are multiplied in addition to the effect of 1.

[0204] 6. The display method of X-ray projection images for medical useparticularly defines the projection interested area of theabove-mentioned display methods into a dental arch area. Therefore, theabove-mentioned effects can be achieved for displaying the X-rayprojection image of the dental arch area.

[0205] 7. According to the display method of X-ray projection images formedical use, in case that the projection interested area is a dentalarch area, the projection plane is arranged to be parallel to a risingdirection of a tooth or a projecting direction of a dental root in thedental arch area. The rising direction of a tooth isn't always adirection perpendicular to an articulation surface of the dental arch,namely a direction orthogonal to a projecting direction of X-ray.Therefore, if a projection plane is normally set, an X-ray projectionimage showing a rising direction of a dental tooth at an angle isobtained and the accurate length of the tooth in a rising length isn'tshown in the image. However, if the projection plane is parallel to therising direction of the tooth, the rising direction of the tooth isaccurately shown on the image, thus improving convenience.

[0206] 8. In the display method of X-ray projection images for medicaluse, when the projection interested area is a dental arch area, arotation center of a projection plane which is rotatively moved isfixed. In such a manner, even in a method a local X-ray CT method isapplied to panoramic images, when a rotation center of a rotary arm isfixed at the time of projection, control is facilitated. Otherwise, arotary arm may be moved in case of projection.

[0207] 9. In the display method of X-ray projection images for medicaluse, a rotation center of the projection plane is transferred in apredetermined pattern in the above-mentioned display methods 5-7 whereinthe projection interested area is a dental arch area. In this case, if arotation center of a rotary arm isn't fixed during projection when thelocal X-ray CT method is applied to panoramic images, clear X-rayprojection images can be obtained by conforming radiating conditions andprojecting conditions. If a rotation center of a rotary arm is fixedduring projection, obstacle shades such as a neck bone can beeliminated.

[0208] 10. In the display method of X-ray projection images for medicaluse, a method characterized in that a projection interested area is setand three dimensional X-ray absorption coefficient data in the area areprojected is applied to produce X-ray panoramic images. Therefore,images without obstacle shades are also obtained for X-ray panoramicimages.

[0209] The projection plane for producing X-ray panoramic images iscurved unlike the flat projection plane in 1. If an object is a dentalarch, the curved projection plane is a curved plane binding thecenterline of the dental arch. These X-ray panoramic images aren't onlyused for a dental arch in the dental field, but they include X-rayprojection images used for other medical field such as diagnosis of ribbone in addition to an otolaryngology area, a dental surgery area, and amaxillo facial area to obtain images by sequentially projecting on thecurved projection plane.

[0210] 11. In the display method of X-ray projection images for medicaluse, the display method characterized in that a projection interestedarea is divided into projection interested layer and the X-rayprojection images obtained by three dimensional X-ray absorptioncoefficient data in the projection interested layer are selectivelydisplayed is applied to X-ray panoramic images. Therefore, as to X-raypanoramic images, where a dental root exists from a cheek to a tonguecan be understood.

[0211] 12. The display apparatus of X-ray projection images for medicaluse is to achieve the above-mentioned display methods 1 and 2.Therefore, it has the same effect as 1 and 2. Further, the X-rayprojection images which have been once stored are read out to bedisplayed so that the X-ray projection images aren't required to beproduced each time. Therefore, the X-ray projection images can bepromptly displayed, thus preventing an operator from being annoyed towait for a display required for diagnosis.

[0212] 13-15. The display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display methods 3-5.Therefore, it has the same effects as 3-5. Further, the X-ray projectionimages which have been once stored are read out to be displayed so thatthe X-ray projection images aren't required to be produced each time.Therefore, the X-ray projection images can be promptly displayed, thuspreventing an operator from being annoyed to wait for display requiredfor diagnosis.

[0213] 16-21. The display apparatus of X-ray projection images formedical use is to achieve the above-mentioned display methods 6-11.Therefore, it has the same effects as 6-11.

[0214] 22. In the X-ray CT apparatus for medical use, an imageconstruction means having an X-ray generator and an X-ray detector iscombined with the display method 15. It achieves the display method 5and has the effect of 5 as a medical X-ray CT apparatus.

[0215] 23. The X-ray CT apparatus for medical use achieves the displaymethod in 6 and has the effect of 6.

[0216] 24. The X-ray CT apparatus for medical use is for obtaining X-raypanoramic images, achieves the display methods in 10 and 11 and has theeffects of 10 and 11.

[0217] 25. According to the X-ray CT apparatus for medical use, inaddition to the effects of the apparatus 22-24, a rotation center of arotary arm is moved, not being fixed during X-ray radiation. For exampleif the a rotation center of radiated X-rays is moved along an envelopecurve, X-rays can be radiated on a dental arch from a directionsubstantially perpendicular to a tooth, thereby achieving valuableimages for diagnosis without obstacle shades.

[0218] 26. According to the X-ray CT apparatus for medical use, inaddition to the effects of the apparatus 22-24, comparing with theapparatus 25, a rotation center of a rotary arm is fixed, an object isgradually moved during X-ray radiation, thus a rotation center of theradiated X-rays is relatively moved. In addition to the same effect as25, more precise X-ray radiation can be accomplished because therotation center of the rotary arum isn't moved. More accurate threedimensional X-ray absorption coefficient data can be obtained from thusobtained transmitted data and as the result more accurate X-rayprojection image can be also obtained.

[0219] 27. According to the X-ray CT apparatus for medical use, inaddition to the effects of the apparatus 22-26, X-rays are radiatedwhile varying a rotational speed of a rotary arm. Therefore, densitycompensation can be executed according to the radiated tooth, thusobtaining better X-ray projection images.

[0220] 28. According to the X-ray CT apparatus for medical use, inaddition to the effects of the apparatus 22-27, like the apparatus 27,density compensation can be executed according to the radiated tooth,thus obtaining better X-ray projection images.

[0221] 29. According to the X-ray CT apparatus for medical use, inaddition to the effects of the apparatus 22-28, inclination of a chairfor holding an object is adjusted in such a manner that an X-rayradiating direction becomes, for example, perpendicular to a risingdirection of a tooth, thereby obtaining transmitted images withoutinclining the rising direction of a tooth. Accordingly, better X-rayprojection images reflecting the rising direction of a tooth can beobtained.

[0222] 30-40. The recording medium saves a program to achieve thedisplay method 1-11. When the medium reads the program and is attachedto an apparatus capable of carrying out the program, the display method1-11 is achieved and the effect of 1-11 is brought out.

1. A display method of an X-ray projection image for medical use whichis obtained by rotating a direction in which the X-ray projection imageis to be projected based on three dimensional X-ray absorptioncoefficient data previously produced by an X-ray CT, the X-ray CT beingcarried out by turning an X-ray generator and an X-ray detector, facingeach other, around an object to be examined, said method comprising thesteps of: a) setting a projection interested area to construct the X-rayprojection image of the object on an image layer with an extendingpredetermined thickness perpendicular to a radiation plane defined byrotational irradiation of an X-ray beam for said X-ray CT, saidprojection interested area being comprised of said three dimensionalX-ray absorption coefficient data; b) producing the X-ray projectionimage by projecting said three dimensional X-ray absorption coefficientdata existing in said projection interested area with respect to aprojection plane intersecting said radiation plane; and c) displayingthus obtained X-ray projection image for medical use.
 2. A displaymethod of an X-ray projection image for medical use, which is obtainedby rotating a direction in which the X-ray projection image is to beprojected based on three dimensional X-ray absorption coefficient datapreviously produced by an X-ray CT, the X-ray CT being carried out byturning an X-ray generator and an X-ray detector, facing each other,around an object to be examined, said method comprising the steps of: a)setting a projection interested area to construct the X-ray projectionimage of the object on an image layer with an extending predeterminedthickness perpendicular to a radiation plane defined by rotationalirradiation of an X-ray beam for said X-ray CT, said projectioninterested area being comprised of said three dimensional X-rayabsorption coefficient data; b) producing the X-ray projection image byprojecting said three dimensional X-ray absorption coefficient dataexisting in said projection interested area with respect to a projectionplane intersecting said radiation plane; and c) displaying in array thusobtained X-ray projection image for medical use.
 3. A display method ofan X-ray projection image for medical use which is obtained by rotatinga direction in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject o be examined, said method comprising the steps of: a) setting aprojection interested area to construct the x-ray projection image ofthe object on an image layer with an extending predetermined thicknessperpendicular to a radiation plane defined by rotational irradiation ofan X-ray beam for said X-ray CT, said projection interested area beingcomprised of said three dimensional X-ray absorption coefficient data;b) producing the X-ray projection image by projecting said threedimensional X-ray absorption coefficient data existing in saidprojection interested area with respect to a projection planeintersecting radiation plane; and c) displaying in sequence thusobtained X-ray projection image for medical use.
 4. A display method ofan X-ray projection image for medical use which is obtained by rotatinga direction in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject to be examined, said method comprising the steps of: a) setting aprojection interested area to construct the X-ray projection image ofthe object on an image layer with an extending predetermined thicknessperpendicular to a radiation plane defined by rotational irradiation ofan X-ray beam for said X-ray CT, said projection interested area beingcomprised of said three dimensional X-ray absorption coefficient data;b) dividing said projection interested area into plural projectioninterested layers neighboring each other in an X-ray radiatingdirection; c) producing the X-ray projection image by projecting saidthree dimensional X-ray absorption coefficient data existing in saidprojection interested area with respect to a projection planeintersecting radiation plane; and d) displaying selectively thusobtained X-ray projection image for medical use.
 5. A display method ofan X-ray projection image for medical use which is obtained by rotatinga direction in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject to be examined, said method comprising the steps of: a) setting acurved projection interested area to construct the X-ray projectionimage of the object on an image layer with an extending predeterminedthickness perpendicular to a radiation plane defined by rotationalirradiation of an X-ray beam for said X-ray CT, said projectioninterested area being comprised of one of said three dimensional X-rayabsorption coefficient data; b) dividing said projection interested areainto plural projection interested layers neighboring each other in anX-ray radiating direction; c) producing the X-ray projection image byprojecting said three dimensional X-ray absorption coefficient dataexisting in the optional one projection interested layer or the pluralprojection interested layers with respect to a projection planeintersecting said radiation plane; and d) displaying the object in arotational manner by showing selectively or in sequence thus obtainedX-ray projection image for medical use.
 6. The display method of anX-ray projection image for medical use as set forth in claim 5, whereinsaid projection interested area is a dental arch area.
 7. The displaymethod of an X-ray projection image for medical use as set forth inclaim 6, wherein said projection plane is arranged to be parallel to arising direction of a tooth or a projecting direction of a dental root.8. The display method of an X-ray projection image for medical use asset forth in any one of claims 5-7, wherein a rotation center of saidprojection plane which moves rotatively is fixed.
 9. The display methodof an X-ray projection image for medical use as set forth in any one ofclaims 5-7, wherein a rotation center of said projection plane whichmoves rotatively is moved in a predetermined form.
 10. A display methodof an X-ray projection image for medical use for showing an X-raypanoramic image based on three dimensional X-ray absorption coefficientdata previously produced by an X-ray CT, the X-ray CT being carried outby turning an X-ray generator and an X-ray detector, facing each other,around an object to be examined, said method comprising the steps of: a)setting a projection interested area to construct the X-ray projectionimage of the object on an image layer with an extending predeterminedthickness perpendicular to a radiation plane defined by rotationalirradiation of an X-ray beam for said X-ray CT, said projectioninterested area being comprised of said three dimensional X-rayabsorption coefficient data; b) producing the X-ray panoramic image byprojecting said three dimensional X-ray absorption coefficient dataexisting in said projection interested area on a projection plane; andc) displaying thus obtained X-ray panoramic image for medical use.
 11. Adisplay method of an X-ray projection image for medical use for showingan X-ray panoramic image based on three dimensional X-ray absorptioncoefficient data previously produced by an X-ray CT, the X-ray CT beingcarried out by turning an X-ray generator and an X-ray detector, facingeach other, around an object to be examined, said method comprising thesteps of: a) setting a projection interested area to construct the X-rayprojection image of the object on an image player with an extendingpredetermined thickness perpendicular to a radiation plane defined byrotational irradiation of an X-ray beam for said X-ray CT, saidprojection interested area being comprised of said three dimensionalX-ray absorption coefficient data; b) dividing said projectioninterested area into plural projection interested layers neighboringeach other in an X-ray radiating direction; c) producing the X-raypanoramic image by projecting said three dimensional X-ray absorptioncoefficient data existing in the optional one projection interestedlayer or the plural projection interested layers on a projection plane;and d) displaying selectively thus obtained X-ray panoramic image formedical use.
 12. A display apparatus of an X-ray projection image formedical use which is obtained by rotating a direction in which the X-rayprojection image is to be projected based on three dimensional X-rayabsorption coefficient data previously produced by an X-ray CT, theX-ray CT being carried out by turning an X-ray generator and an X-raydetector, facing each other, around an object to be examined, wherein: aprojection interested area to construct the X-ray projection image ofthe object is set on an image layer with an extending predeterminedthickness perpendicular to a radiation plane defined by rotationalirradiation of an X-ray beam for said X-ray CT, said projectioninterested area is comprised of one of said three dimensional X-rayabsorption coefficient data, and the X-ray projection image is producedby projecting said three dimensional X-ray absorption coefficient dataexisting in said projection interested area with respect to a projectionplane intersecting said radiation plane; and said display apparatuscomprising: a) a memory means for storing thus obtained X-ray projectionimage; and b) a display means for showing in array thus obtained X-rayprojection image read from said memory means.
 13. A display apparatus ofan X-ray projection image for medical use which is obtained by rotatinga direction in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject to be examined, wherein: a projection interested area toconstruct the X-ray projection image of the object is set on an imagelayer with an extending predetermined thickness perpendicular to aradiation plane defined by rotational irradiation of an X-ray beam forsaid X-ray CT, said projection interested area is comprised of one ofsaid three dimensional X-ray absorption coefficient data, and the X-rayprojection image is produced by projecting said three dimensional X-rayabsorption coefficient data existing in said projection interested areawith respect to a projection plane intersecting said radiation plane;and said display apparatus comprising: a) a memory means for storingthus obtained X-ray projection image; and b) a display means for showingthe object in a rotational manner by sequentially showing thus obtainedX-ray projection image read out of said memory means.
 14. A displayapparatus of an X-ray projection image for medical use which is obtainedby rotating a direction in which the X-ray projection image is to beprojected based on three dimensional X-ray absorption coefficient datapreviously produced by an X-ray CT, the X-ray CT being carried out byturning an X-ray generator and an X-ray detector, facing each other,around an object to be examined, wherein: a projection interested areato construct the X-ray projection image of the object is set on an imageplayer with an extending predetermined thickness perpendicular to aradiation plane defined by rotational irradiation of an X-ray beam forsaid X-ray CT, said projection interested area is comprised of one ofsaid three dimensional X-ray absorption coefficient data, saidprojection interested area is divided into plural projection interestedlayers neighboring each other in an X-ray radiating direction, and theX-ray projection image is produced by projecting said three dimensionalX-ray absorption coefficient data existing in the optional oneprojection interested layer or the plural projection interested layerswith respect to a projection plane intersecting said radiation plane;and said display apparatus comprising: a) a memory means for storingthus obtained X-ray projection image: and b) a display means selectivelyshowing said X-ray projection interested area read out of said memorymeans or said X-ray projection image.
 15. A display apparatus of anX-ray projection image for medical use which is obtained by rotating adirection in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject to be examined, wherein: a projection interested area toconstruct the X-ray projection image of the object is set on an imagelayer with an extending predetermined thickness perpendicular to aradiation plane defined by rotational irradiation of an X-ray beam forsaid X-ray CT, said projection interested area is comprised of one ofsaid three dimensional X-ray absorption coefficient data, and saidprojection interested area is divided into plural projection interestedlayers neighboring each other in an X-ray radiating direction; and saiddisplay apparatus comprising: a) a selection means for selecting any oneprojection interested layer or plural projection interested layersneighboring each other; b) a memory means for storing the X-rayprojection image produced by projecting the three dimensional X-rayabsorption coefficient data existing in the selected projectioninterested layer with respect to a projection plane intersecting saidradiation plane; and c) a display means showing the object in arotational manner by selectively or in sequence showing said X-rayprojection image read out of said memory means.
 16. The displayapparatus of an X-ray projection image for medical use as set forth inany one of claims 12-15, wherein said projection interested area is adental arch area.
 17. The display apparatus of an X-ray projection imagefor medical use as set forth in claim 16, wherein said projection planeis arranged to be parallel to a rising direction of a tooth or aprojecting direction of a dental root.
 18. The display apparatus of anX-ray projection image for medical use as set forth in any one of claims16 and 17, wherein a rotation center of said projection plane whichmoves rotatively is fixed.
 19. The display apparatus of an X-rayprojection image for medical use as set forth in any one of claims 16and 17, wherein a rotation center of said projection plane which movesrotatively is moved in a predetermined form.
 20. A display apparatus ofan X-ray projection image for showing an X-ray panoramic image based onthree dimensional X-ray absorption coefficient data previously producedby an X-ray CT, the X-ray CT being carried out by turning an X-raygenerator and an X-ray detector, facing each other, around an object tobe examined wherein: a projection interested area to construct the X-rayprojection image of the object is set on an image layer with anextending predetermined thickness perpendicular to a radiation planedefined by rotational irradiation of an X-ray beam for said X-ray CT,said projection interested area is comprised of one of said threedimensional X-ray absorption coefficient data, and the X-ray panoramicimage is produced by projecting said three dimensional X-ray absorptioncoefficient data existing in said projection interested area on aprojection plane; and said display apparatus comprising: a) a memorymeans for storing thus obtained X-ray panoramic image; and b) a displaymeans for showing thus obtained X-ray panoramic image read from saidmemory means.
 21. A display apparatus of an X-ray projection image forshowing an X-ray panoramic image based on three dimensional X-rayabsorption coefficient data previously produced by an X-ray CT, theX-ray CT being carried out by turning an X-ray generator and an X-raydetector, facing each other, around an object to be examined wherein: aprojection interested area to construct the X-ray projection image ofthe object is set on an image layer with an extending predeterminedthickness perpendicular to a radiation plane defined by rotationalirradiation of an X-ray beam for said X-ray CT, said projectioninterested area is divided into plural projection interested layersneighboring each other in an X-ray radiating direction, and the X-raypanoramic image is produced by projecting said three dimensional X-rayabsorption coefficient data existing in the optional one projectioninterested layer or the plural projection interested layers on aprojection plane; and said display apparatus comprising: a) a memorymeans for storing thus obtained X-ray panoramic image: and b) displaymeans for selectively showing said X-ray panoramic image read out ofsaid memory means.
 22. An X-ray CT apparatus for medical use having arotary arm with an X-ray generator and an X-ray detector facing eachother for radiating X-rays around an object to be examined and having animage construction means which constructs an X-ray projection image on adesired projection plane based on the object's three dimensional X-rayabsorption coefficient data obtained by the output of the X-raydetector; wherein a projection interested area to construct the X-rayprojection image of the object is set on an image layer with anextending predetermined thickness perpendicular to a radiation planedefined by rotational irradiation of an X-ray beam, said projectioninterested area is comprised of one of said three dimensional X-rayabsorption coefficient data, and said projection interested area isdivided into plural projection interested layers neighboring each otherin an X-ray radiating direction; and said X-ray CT apparatus comprising:a) a selection means for selecting one projection interested layer orplural projection interested layers neighboring each other; b) a memorymeans for storing the X-ray projection image produced by projecting thethree dimensional X-ray absorption coefficient data existing in saidselected projection interested layer on a projection plane intersectingsaid radiation plane; and c) a display means showing the object inrotational manner by showing selectively or in sequence said X-rayprojection image read out of said memory means.
 23. The X-ray CTapparatus for medical use as set forth in claim 22, wherein saidprojection interested area is a dental arch area.
 24. An X-ray CTapparatus for medical use having a rotary arm with an X-ray generatorand an X-ray detector facing each other for radiating X-rays around anobject to be examined and having an image construction means whichconstructs an X-ray panoramic image on a desired projection plane basedon the object's three dimensional X-ray absorption coefficient dataobtained by the output of the X-ray detector; wherein a projectioninterested area to construct an X-ray projection image of the object isset on an image layer with an extending predetermined thicknessperpendicular to a radiation plane defined by rotational irradiation ofan X-ray beam, said projection interested area is comprised of one ofsaid three dimensional X-ray absorption coefficient data, and saidprojection interested area is divided into plural projection interestedlayers neighboring each other in an X-ray radiating direction; and saidX-ray CT apparatus comprising: a) a selection means for selecting oneprojection interested layer or plural projection interested layersneighboring each other; b) a memory means for storing the X-raypanoramic image obtained by projecting the three dimensional X-rayabsorption coefficient data in the selected projection interested layeron a projection plane: and c) display means for showing the X-raypanoramic image in array read out of said memory means.
 25. The X-ray CTapparatus for medical use as set forth in any one of claims 22-24,wherein a rotation center of said rotary arm is moved during X-rayradiation by the minute.
 26. The X-ray CT apparatus for medical use asset forth in any one of claims 22-24, wherein a chair holding saidobject is moved accompanied with a turning position of said rotary armduring X-ray radiation.
 27. The X-ray CT apparatus for medical use asset forth in any one of claims 22-26, wherein a rotational velocity ofsaid rotary arm is varied during X-ray radiation.
 28. The X-ray CTapparatus for medical use as set forth in any one of claims 22-27,wherein an X-ray tube voltage and/or an X-ray tube current is/are variedaccording to a rotational position of said rotary arm.
 29. The X-ray CTapparatus for medical use as set forth in any one of claims 22-28,wherein a chair holding the object is adjustably tilted.
 30. A recordingmedium for recording a program to achieve a method of displaying anX-ray projection image for medical use which is obtained by rotating adirection in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject to be examined; said program comprising the steps of: a) settinga projection interested area to construct the X-ray projection image ofthe object on an image layer with an extending predetermined thicknessperpendicular to a radiation plane defined by rotational irradiation ofan X-ray beam for said X-ray CT, said projection interested area beingcomprised of said three dimensional X-ray absorption coefficient data;b) producing the X-ray projection image by projecting said threedimensional X-ray absorption coefficient data existing in saidprojection interested area with respect to a projection planeintersecting said radiation plane; and c) displaying thus obtained X-rayprojection image for medical use.
 31. A recording medium for recording aprogram to achieve a method of displaying an X-ray projection image formedical use which is obtained by rotating a direction in which the X-rayprojection image is to be projected based on three dimensional X-rayabsorption coefficient data previously produced by an X-ray CT, theX-ray CT being carried out by turning an X-ray generator and an X-raydetector, facing each other, around an object to be examined; saidprogram comprising the steps of: a) setting a projection interested areato construct the X-ray projection image of the object on an image layerwith an extending predetermined thickness perpendicular to a radiationplane defined by rotational irradiation of an X-ray beam for said X-rayCT, said projection interested area being comprised of said threedimensional X-ray absorption coefficient data; b) producing the X-rayprojection image by projecting said three dimensional X-ray absorptioncoefficient data existing in said projection interested area withrespect to a projection plane intersecting said radiation plane; and c)displaying in array thus obtained X-ray projection image for medicaluse.
 32. A recording medium for recording a program to achieve a methodof displaying an X-ray projection image for medical use which isobtained by rotating a direction in which the X-ray projection image isto be projected based on three dimensional X-ray absorption coefficientdata previously produced by an X-ray CT, the X-ray CT being carried outby turning an X-ray generator and an X-ray detector, facing each other,around an object to be examined, said program comprising the steps of:a) setting a projection interested area to construct the X-rayprojection image of the object on an image layer with an extendingpredetermined thickness perpendicular to a radiation plane defined byrotational irradiation of an X-ray beam for said X-ray CT, saidprojection interested area being comprised of said three dimensionalX-ray absorption coefficient data; b) producing the X-ray projectionimage with by projecting said three dimensional X-ray absorptioncoefficient data existing in said projection interested area withrespect to a projection plane intersecting said radiation plane; and c)displaying the object in a rotational manner by showing in sequence thusobtained X-ray projection image for medical use.
 33. A recording mediumfor recording a program to achieve a method of displaying an X-rayprojection image for medical use which is obtained by rotating adirection in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject to be examined; said program comprising the steps of: a) settinga projection interested area to construct the X-ray projection image ofthe object on an image layer with an extending predetermined thicknessperpendicular to an X-ray radiation plane defined by rotationalirradiation of an X-ray beam for said X-ray CT, said projectioninterested area being comprised of said three dimensional X-rayabsorption coefficient data; b) dividing said projection interested areainto plural projection interested layers neighboring each other in anX-ray radiating direction; c) producing the X-ray projection image byprojecting said three dimensional X-ray absorption coefficient dataexisting in the optional one projection interested layer or the pluralprojection interested layers with respect to a projection planeintersecting said radiation plane; and d) displaying selectively thusobtained X-ray projection image for medical use.
 34. A recording mediumfor recording a program to achieve a method of displaying an X-rayprojection image for medical use which is obtained by rotating adirection in which the X-ray projection image is to be projected basedon three dimensional X-ray absorption coefficient data previouslyproduced by an X-ray CT, the X-ray CT being carried out by turning anX-ray generator and an X-ray detector, facing each other, around anobject to be examined; said program comprising the steps of: a) settinga curved projection interested area to construct the X-ray projectionimage of said object on an image layer with an extending predeterminedthickness perpendicular to a radiation plane defined by rotationalirradiation of an X-ray beam for said X-ray CT, said projectioninterested area being comprised of said three dimensional X-rayabsorption coefficient data; b) dividing said projection interested areainto plural projection interested layers neighboring each other in anX-ray radiating direction; c) producing the X-ray projection image byprojecting said three dimensional X-ray absorption coefficient dataexisting in the optional one projection interested layer or the pluralprojection interested layers with respect to a projection planeintersecting said radiation plane; and d) displaying the object in arotational manner by selectively or in sequence showing thus obtainedX-ray projection image for medical use.
 35. The recording medium as setforth in claim 34, wherein said projection interested area is a dentalarch area.
 36. The recording medium as set forth in claim 35, whereinsaid projection plane is arranged to be parallel to a rising directionof a tooth or a projecting direction of a dental root.
 37. The recordingmedium as set forth in any one of claims 34-36, wherein a rotationcenter of said projection plane which moves rotatively is fixed.
 38. Therecording medium as set forth in any one of claims 34-36, wherein arotation center of said projection plane which moves rotatively is movedin a predetermined form.
 39. A recording medium for recording a programto achieve a method of displaying an X-ray panoramic image for medicaluse based on three dimensional X-ray absorption coefficient datapreviously produced by an X-ray CT, the X-ray CT being carried out byturning an X-ray generator and an X-ray detector, facing each other,around an object to be examined, said program comprising the steps of:a) setting a projection interested area to construct the X-rayprojection image of the object on an image layer with an extendingpredetermined thickness perpendicular to an X-ray radiation planedefined by rotational irradiation of an X-ray beam for said X-ray CT,said projection interested area being comprised of said threedimensional X-ray absorption coefficient data; b) producing the X-raypanoramic image by projecting said three dimensional X-ray absorptioncoefficient data existing in said projection interested area withrespect to a projection plane; and c) displaying thus obtained X-raypanoramic image for medical use.
 40. A recording medium for recording aprogram to achieve a method of displaying an X-ray panoramic image formedical use based on three dimensional X-ray absorption coefficient datapreviously produced by an X-ray CT, the X-ray CT being carried out byturning an X-ray generator and an X-ray detector, facing each other,around an object to be examined, said program comprising the steps of:a) setting a projection interested area to construct the X-rayprojection image of the object on an image layer with an extendingpredetermined thickness perpendicular to an X-ray radiation planedefined by rotational irradiation of an X-ray beam for said X-ray CT,said projection interested area being comprised of said threedimensional X-ray absorption coefficient data; b) dividing theprojection interested area into plural projection interested layersneighboring each other in an X-ray radiating direction; c) producing anX-ray panoramic image by projecting said three dimensional X-rayabsorption coefficient data existing in the one optional projectioninterested layer or the plural projection interested layers with respectto a projection plane; and d) displaying selectively thus obtained X-raypanoramic image for medical use.